1. Field of the Invention
The present invention relates to a helicopter rotor equipped with flaps.
2. Description of the Related Art
A helicopter is generally maneuvered by controlling a pitch angle in a blade root of a main rotor by means of actuators, swash plates, link mechanisms, and the like.
The helicopter rotor is a high-speed rotatable element. Additionally the vibration level of the helicopter rotor is considerably high, so that a highly accurate mechanism is required for controlling the pitch angle of a blade. The aerodynamic moment generated about the feathering axis over the whole blade is large. In order to change the pitch angle of the blade against the mass of the rotor and control mechanism, a high-power actuator and a high-power hydraulic mechanism are required. Such requirements result in difficulty in reducing the weight of the structure of the helicopter.
As a countermeasure against such drawbacks, a method is proposed in which a relatively small flap is attached to the vicinity of a blade tip where a dynamic pressure is sufficiently high, and the pitch angle of a blade is controlled by utilizing a large aerodynamic force generated in that portion (for example, Japanese Unexamined Patent Publication JPA 6-107293 (1994) and U.S. Pat. Nos. 3,077,934, 3,129,769, 3,589,831, and 4,461,611). The attachment of a flap to the vicinity of a blade tip enables the pitch angle to be controlled with a small control force, so that the whole control mechanism can be reduced in weight as well as in size.
In a prior art helicopter rotor, a large centrifugal force due to the high-speed revolution of the rotor acts on the center of gravity of a flap. In the vicinity of the blade tip, for example, the centrifugal force has a large value which is about 1,000 times as large as the gravity.
FIG. 4 is a partial perspective view showing an example of a prior art of helicopter rotor. A blade 1 of the rotor is attached to a main rotor shaft 2, and rotated at a high speed in a counterclockwise direction when viewed from above. In the vicinity of a tip of the blade 1, a flap 3 having a wing shape in section is pivotally supported with a hinge (not shown) so as to be angularly displaceable and at the end a part of a rear end of the blade 1 is cut out. A hinge line 4 indicating the center of the angular displacement of the hinge is set in parallel to the spanwise direction of the blade 1, and crosses a straight line L passing through the rotation center C of the rotor and the center of gravity G of the flap, at about 2.5 degrees.
The centrifugal force CF acting on the center of gravity G of the flap is split into a component force F1 alone the hinge line 4, a component force F2 alone a wing chord direction of the flap 3 in a plane perpendicular to the hinge line 4, and a component force F3 alone a wing thickness direction of the flap 3. The component forces F1 and F2 are supported by the hinge, but the component force F3 acts as a hinge moment about the hinge line 4 so as to angularly displace the flap 3.
In this way, a centrifugal force affects a hinge moment of the flap, and the following problems arise:
1) The moment due to the centrifugal force serves as additional load, so that the load conditions for the flap control mechanism become more severe, and it is difficult to design a compact mechanism. PA0 2) Since it is necessary to increase the output power of a flap control actuator, it is difficult to design a compact actuator and a compact hydraulic system. PA0 3) A component of the centrifugal force which is along a flap thickness direction causes torsion or flapping in the blade, so that there occurs an aeroelastic phenomenon which may cause an unexpected change in an effective angle of attack of a blade element, and the aeroelastic phenomenon adversely affects the flight performance, the flight characteristics, and the vibration level.